The present invention relates to a nonlubricated screw fluid machine in which engaging male and female rotors are rotatably mounted in a casing thereof with no lubricating oil being supplied within the casing and, more particularly, to a screw fluid machine including a rotor with a tooth profile that is preferably used in a nonlubricated screw compressor or a nonlubricated screw vacuum pump.
A conventional type of nonlubricated screw fluid machine provides air containing no oil component since no oil is supplied during the rotation of a pair of engaging rotors. Such nonlubricated screw fluid machines are, therefore, widely used in various fields related to semiconductor manufacturing, the food industry and biotechnology.
In the nonlubricated screw fluid machine of the type described above, engaging male rotors are rotatably disposed in a casing with the engagement between the rotors being such that substantially constant small gaps being formed therebetween by virtue of a synchronizing device provided on the rotor shaft portion disposed outside the casing. In these machines, in order to prevent any deterioration in sealing performance due to the presence of the small gaps between the rotors, the rotational speed of the rotors must be several times that of oil cooling type rotors.
As a result, the temperature of the rotors is raised to several hundred degrees during operation, and thermal expansion also becomes greater with respect to the shape of the rotor at room temperature when the rotors are stopped. It is, therefore, necessary that the two rotors rotate so as not to interfere with each other in view of the thermal expansion of the two rotors. In particular, since the inlet side of the rotor and the outlet side thereof are different in temperature distribution and thermal expansion, the diameter of the outlet (discharging) side of the rotor in the direction of the shaft of the rotor is conventionally smaller, while the diameter of the inlet (suction) side is larger so that no mutual interference takes place by virtue of the thus-realized tapered shape. This is accomplished by, for example, machining or a corrosion method.
An arrangement of the aforementioned type is disclosed, for example, in Japanese Patent Unexamined Publication No. 59-208077.
Since the above-tapered rotors are formed conventionally with an inclination corresponding to the difference in the radius of tooth-bottom thereof on the basis of the difference between the inlet side and the outlet side in the temperature distribution, the width of each bottom portion becomes larger at the outlet side than at the inlet side after the machining has been completed.
As a result, the rotors may interfere with each other at their bottom portions when the nonlubricated screw fluid machine is operated, causing a problem of possible interference between the rotors.